1. Field of the Invention
The present invention relates to a hydrodynamic bearing device for a spindle motor used in an information recording and reproducing apparatus such as a hard disk drive (hereinafter referred to as HDD), and more particularly relates to a technique for fixing a flange or a hub to a shaft which is nitrided to improve hardness and wear resistance.
2. Background Information
Various kinds of information recording and reproducing apparatus in which a HDD is installed have undergone reductions in weight, thickness, and overall size in recent years to make them more portable. However, as the size is reduced, the large memory capacity must be maintained or improved, and impact resistance also must be increased. Hydrodynamic bearing devices, which have high rotation accuracy and excellent impact resistance, have been used as the bearings for spindle motors used in these HDDs.
With a hydrodynamic bearing, as the motor becomes thinner, the length of the fixing portion between the shaft and a flange or hub (the fixed member) decreases, and stronger fastening becomes necessary. The most common ways for fastening the fixing portion are laser welding, a combination of adhesive bonding and press fitting, and so forth.
The shaft that is used in a hydrodynamic bearing device is, for example, made of austenite stainless steel, which has a coefficient of linear expansion close to that of the sleeve material, and Patent Document 1 discloses a technique for increasing hardness and wear resistance by forming a nitriding layer on the surface of the shaft.                Patent Document 1: Japanese Laid-Open Patent Application H10-89345        Patent Document 2: Japanese Laid-Open Patent Application 2004-291851        Patent Document 3: Japanese Laid-Open Patent Application H3-237278        
With the conventional configuration discussed above, however, if, for example, a mating component with a flange or other such fixed member is laser welded at the surface of a shaft that has undergone a nitriding treatment, because the nitrided part is hard and has low ductility, a problem is that cracks may develop at the weld site, preventing a proper weld from being obtained. When an adhesive is used for the fixing, a problem is that adhesion deteriorates due to temperature changes and so forth because the nitride layer, which has a different coefficient of linear expansion, is interposed between the shaft and the flange. When press fitting, caulking, or screws are used for the fixing, the surface undergoes plastic deformation, so the nitride layer on the surface may separate, and this tends to be a source of fouling and foreign matter adhesion.